Relay Switch Including an Energy Detection Circuit

ABSTRACT

A semiconductor relay switch having two data ports receiving incoming signals and a power supply terminal receiving a power supply voltage is responsive to a power supply voltage level and an energy level of the incoming signals to open and close its conduction paths. The relay switch is open when a valid power supply level is detected and when there is no supply power on the power supply terminal but a high energy level is detected in the incoming signals. The relay switch is closed to allow conduction between the two data ports only when there is no power supply voltage on the power supply terminal and an energy level below a predetermined threshold is detected in the incoming signals. In one embodiment, the semiconductor relay switch includes a main conduction switch circuit, an energy detect circuit and a control signal generator.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of application Ser. No. 11/151,033,filed Jun. 13, 2005, entitled “Relay Switch Including an EnergyDetection Circuit” of the same inventors hereof, which application isincorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The invention relates to relay switches and, in particular, to a relayswitch that is turn on or off based in part on energy detection.

DESCRIPTION OF THE RELATED ART

Power Over Ethernet (PoE) technology has been developed to allow userdevices, such as IP telephones, wireless LAN Access Points and otherappliances, to receive power as well as data over existing networkcabling, without needing to modify the existing Ethernet infrastructure.

In general, a PoE network is formed by a Power Sourcing Equipment (PSE)supplying power and a Powered Device (PD) receiving and utilizing thepower. A PSE, typically a midspan network device, injects power onto thetwisted pair cables forming the local area network. At the other end ofthe cables, the power is used to run the Powered Devices so that noadditional source of power needs to be provided to the Powered Devices.The Power Sourcing Equipment thereby provides on the same cable bothpower and data signals to the Powered Devices. When PoE technology isapplied to an existing network, the network may include midspan networkdevices that cannot supply power or user devices that are not PoEcompliant and cannot receive power. Therefore, two issues need to beaddressed.

First, when a Power Sourcing Equipment is installed in a network, thePSE must determine whether user devices connected to the PSE arecompatible with the PoE technology. Thus, a discovery process, run fromthe PSE, is typically carried out to examine the cable and determine ifa device connected to the PSE is PoE compliant and therefore can receivepower from the PSE. The discovery process ensures that devices that maynot be PoE compliant are not forwarded power and thereby possibly sufferdamage. In addition to determining whether the user device has thecapability to receive power, the discovery process also determineswhether or not a PoE compliant user device actually needs power. This isbecause a user device, even though PoE compliant, may be receiving powerfrom another source, such as a wall outlet, and therefore does notrequire power from the PSE.

Second, a PoE compliant Powered Device may be connected to a networkdevice that is not PoE compliant and cannot supply power. In that case,if the Powered Device does not have an alternate power source, then thePowered Device is inoperative but its presence in the network must notdisrupt or interfere with the normal operation of the network device.

Several methodologies have been purposed for implementing the Power OverEthernet technology in existing networks. Industry standard IEEE 802.3afdescribes one PoE methodology where a PSE runs a discovery process toexamine the cable and determine if a device connected to the PSE iscompliant with the IEEE 802.3af specification. Under the IEEE 802.3afstandard, the PSE applies two small current-limited voltage signalsacross the cable and checks for the presence of a characteristicresistance. Power is provided only when the specified resistance isdetected. Because of the requirement of the characteristic resistance,the IEEE 802.3af specification requires Powered Devices to be built fromscratch to incorporate the characteristic resistance in order to be usedwith Power Sourcing Equipments compliant with the IEEE 802.3af standard.

Other methodologies have been proposed to allow Power Over Ethernettechnology to be readily incorporated in existing user devices orappliances without requiring a complete redesign of the user devices orappliances. For instance, one methodology involves using a relay to loopback a unique sequence of signals for the purpose of discovering PoEcapability. FIG. 1 illustrates a network including a Powered Device anda network device where the signal loop back discovery method is used todiscover PoE capability. Referring to FIG. 1, a network device 1, whichmay or may not be a Power Sourcing Equipment, is connected through acable 3 including at least two twisted wire pairs to a client device 2which is a Powered Device capable of receiving power through thetransmission cable 3. Powered Device 2 can also optionally be providedwith capability to receive power (Vdd) from an external source (node 5),such as a wall outlet 7 through an optional AC adaptor 6.

In the configuration shown in FIG. 1, if network device 1 is a PowerSourcing Equipment (PSE), then PSE 1 will send out control pulses havinga unique sequence to discover whether client devices connected theretocan receive power on the transmission cable and further whether theclient devices needs power. If the unique sequence of control pulses isreturned to PSE 1 on the cable, then PSE 1 determines that the clientdevice connected thereto is PoE compliant and requires power. If nocontrol pulses are returned back, then PSE 1 determines that the clientdevice connected thereto is either non-PoE compliant or does not requirepower.

To implement the signal loop back discovery method, a Powered Device asthe client device 2 will be associated with a relay 8 coupled to cable 3for implementing the signal loop back. Relay 8 is controlled by thepower supply (node 5) of client device 2 to open and close in responsethe voltage level of the power supply. Specifically, if power is appliedto the power node 5, relay 8 is open and the control pulses will not belooped back. If no Vdd is applied to the Power node, relay 8 is closedand conducting. The control pulses from PSE 1 is thereby looped back oncable 3 to the PSE to indicate to the PSE that power needs to beprovided to the powered device through transmission cable 3. When PSE 1receives the unique signature of the control pulses being sent back,then the PSE determines that client device 2 has the capability toreceive power and does not have power. Power is then provided along withdata signals to client device 2 on cable 3.

Implementation of the PoE discovery using a relay as shown in FIG. 1 canbe problematic, particularly in cases where a powered device isconnected to a network device that does not support PoE. In theconfiguration shown in FIG. 1, when the network device 1 is a PSEdevice, the PSE device, upon receipt of the returned signals, willrecognize the signals as the looped back data and discard or ignore thedata accordingly. However, when network device 1 is not a PSE device andwhen Powered Device 2 does not have power, relay 8 will be closed tocause any signals sent by network device 1 to be looped back to thenetwork device. Network device 1 will treat the loop back data packetsas new data and will broadcast or retransmit the data packets in itsnormal option. The loop back and repeated transmission of the same datapackets intended for Powered Device 2 drastically increase the networktraffic and often result in jamming. Thus, in a network configurationwhere a Powered Device is connected to a network device not capable ofsupplying power, the signal loop back discovery method can result injamming as large amount of redundant network traffic is put back intothe network by the relay.

One solution to the aforementioned problem is to use a filter, such asfilter 9 in FIG. 1, to filter out the data packets on the loop backpath. When filter 9 is incorporated, only the control pulses arereturned back on the twisted wire pairs to allow a PSE to recognize theunique signature of the control pulses. Data packets, not part of theunique sequence of signals, are blocked from being looped back. Thus,when the network device does not supply power, filter 9 will preventdata packets sent to the Powered Devices from being returned to thenetwork device.

However, the filter solution has limitations. First, the control pulsetypically has a pulse width of 100 to 150 ns. In a 100 Base-T or1000Base-T Ethernet, the pulse width of the data signals is narrowenough so that a filter can accurately filter out the data signals. Butin a 10Base-T network, the pulse width of the data signals is about50-100 ns and is too close to the pulse width of the control pulses tobe effectively distinguished. Thus, the filter solution does work in a10Base-T network because the filter cannot differentiate between thedata pulses and the control pulses.

Other solutions involve edge counting within certain window todifferentiate the low repetition rate of the control pulses and thetransition rich data packets. However, such technique is very difficultto implement considering the fact that the Powered Device does not havepower. While a positive wave selector can be used to accumulate power, aconventional diode rectifier cannot be used due to the voltage dropacross the diode rectifier where such voltage drop is often significantcompared to the magnitude of the data signals.

An improved method for practicing signal loop back discovery for aPowered Device in a data network is desired.

SUMMARY OF THE INVENTION

According to one embodiment of the present invention, a semiconductorrelay switch having two data ports receiving incoming signals and apower supply terminal receiving a power supply voltage is responsive toa power supply voltage level and an energy level of the incoming signalsto open and close its conduction paths. The relay switch is open when avalid power supply level is detected and when there is no supply poweron the power supply terminal but a high energy level is detected in theincoming signals. The relay switch is closed to allow conduction betweenthe two data ports only when there is no power supply voltage on thepower supply terminal and an energy level below a predeterminedthreshold is detected in the incoming signals. In one embodiment, thesemiconductor relay switch includes a main conduction switch circuit, anenergy detect circuit and a control signal generator.

In one embodiment, a relay switch circuit includes a first data portcoupled to a first pair of wires carrying a first pair of differentialsignals, a second data port coupled to a second pair of wires carrying asecond pair of differential signals, and a power supply terminalreceiving a power supply voltage signal. The relay switch circuitfurther includes an energy detect circuit coupled to measure the energyof incoming signals on the first data port and the second data port. Theenergy detect circuit generates a first energy detect signal and asecond energy detect signal indicative of an energy level of theincoming signals on the respective first and second data ports. Thefirst and second energy detect signals have a first state when an energylevel exceeding a predetermined threshold is measured on the incomingsignals at the respective data port and a second state otherwise. Therelay switch circuit further includes a control signal generator circuitcoupled to receive incoming signals from the first data port and thesecond data port, the power supply voltage signal, and the first andsecond energy detect signals. The control signal generator generates oneor more control signals in response. The control signal generatorderives power for its operation from the incoming signals. Finally, therelay switch circuit includes a switch circuit coupled to the first dataport and the second data port and being controlled by the one or morecontrol signals. The switch circuit is operated to open to isolate thefirst data port from the second data port or close to transmitdifferential signals onto and receive differential signals from thefirst and second data ports.

In operation, the control signal generator generates the one or morecontrol signals to cause the switch circuit to open when the powersupply voltage signal indicates a valid power supply voltage and to openwhen the power supply voltage signal indicates an invalid power supplyvoltage and one of the first and second energy detect signals has thefirst state. The control signal generator generates the one or morecontrol signals to cause the switch circuit to close when the powersupply voltage signal indicates an invalid power supply voltage and bothof the first and second energy detect signals have the second state.

According to another embodiment of the present invention, a method foroperating a relay switch connected between a first data port and asecond data port and receiving a power supply voltage signal includes:measuring the voltage level of the power supply voltage signal,measuring an energy level of incoming signals on either one of the firstand second data ports, opening the relay switch to isolate the first andsecond data ports when the voltage level of the power supply voltagesignal is greater than a predetermined supply voltage threshold level,opening the relay switch to isolate the first and second data ports whenthe voltage level of the power supply voltage signal is equal to orbelow the predetermined supply voltage threshold level and the energylevel of the incoming signals on either one of the first and second dataports is greater than a predetermined energy threshold level, andclosing the relay switch to electrically connect the first and seconddata ports when the voltage level of the power supply voltage signal isequal to or below the predetermined supply voltage threshold level andthe energy level of the incoming signals on either one of the first andsecond data ports is less than or equal to the predetermined energythreshold level.

The present invention is better understood upon consideration of thedetailed description below and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a network including a Powered Device and a networkdevice where the signal loop back discovery method is used to discoverPoE capability.

FIG. 2 illustrates the incorporation of the relay switch of the presentinvention in a Powered Device coupled to a network device via atransmission cable according to one embodiment of the present invention.

FIG. 3 is a block diagram of a relay switch according to one embodimentof the present invention.

FIG. 4 is a circuit diagram of the main conduction switch circuit of therelay switch according to one embodiment of the present invention.

FIG. 5 is a circuit diagram of the energy detect circuit of the relayswitch according to one embodiment of the present invention.

FIG. 6 is a circuit diagram of the control signal generator of the relayswitch according to one embodiment of the present invention.

FIG. 7 is a circuit diagram of a Vdd level generator circuitincorporated in the control signal generator of FIG. 6 according to oneembodiment of the present invention.

FIG. 8 is a circuit diagram of a first generator circuit of the controlsignal generator of FIG. 6 according to one embodiment of the presentinvention.

FIG. 9 is a circuit diagram of a second generator circuit of the controlsignal generator of FIG. 6 according to one embodiment of the presentinvention.

FIG. 10 is a circuit diagram of the energy detect control circuit 1 ofFIG. 6 according to one embodiment of the present invention.

FIG. 11 is a circuit diagram of the energy detect control circuit 2 ofFIG. 6 according to one embodiment of the present invention.

FIG. 12 is a circuit diagram of a first latch circuit which can beincorporated in the control signal generator of FIG. 6 according to oneembodiment of the present invention.

FIG. 13 is a circuit diagram of a second latch circuit which can beincorporated in the control signal generator of FIG. 6 according to oneembodiment of the present invention.

FIG. 14 is a circuit diagram of a first Vdd/energy detect controlcircuit according to one embodiment of the present invention.

FIG. 15 is a circuit diagram of a second Vdd/energy detect controlcircuit according to one embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with the principles of the present invention, asemiconductor relay switch having two data ports receiving incomingsignals and a power supply terminal receiving a power supply voltage isresponsive to a power supply voltage level and an energy level of theincoming signals to open and close its conduction paths. The relayswitch is open when a valid power supply level is detected and whenthere is no supply power on the power supply terminal but a high energylevel is detected in the incoming signals. The relay switch is closed toallow conduction between the two data ports only when there is no powersupply voltage on the power supply terminal and an energy level below apredetermined threshold is detected in the incoming signals. In oneembodiment, the semiconductor relay switch includes a main conductionswitch circuit, an energy detect circuit and a control signal generator.

Importantly, the relay switch of the present invention extracts energyfrom the incoming signals so that the relay switch can operate to closethe conduction paths even when there is no power supply voltage providedto the relay switch. In one embodiment, the relay switch draws powerusing a positive wave selector that does not incur undesired voltagedrop to enable effective operation for the input signals having limitedvoltage range. By accumulating energy for its operation from theincoming signals, the relay switch of the present invention can beoperated without a separate power supply and is therefore useful inapplications where a separate power supply is not available or cannot beprovided, such as when the relay switch is coupled to a Powered Deviceimplementing Power Over Ethernet (PoE) technology.

In one application, the relay switch is coupled to a PoE-compliantPowered Device to facilitate discovery in a network utilizing the signalloop back discovery method described above. When the relay switch of thepresent invention is applied in a Powered Device for implementing signalloop back discovery, the relay switch operates to return only the uniquesequence of control pulses, sent by a Power Sourcing Equipment (PSE) fordiscovery purposes, when the Powered Device does not have supply power.The relay switch will not return incoming signals that are data packets,thereby avoiding jamming a network device connected to the PoweredDevice in the event that the network device is not a PoE compliant PSE.

In one embodiment, the main conduction switch circuit of the relayswitch is constructed using a dual-switching-device configuration foreach conduction path. The dual-switching-device arrangement enables therelay switch to achieve good isolation between the two data ports of therelay switch. Furthermore, in another embodiment, a lowimpedance-shunting path is inserted between each pair of switchingdevices to further improve the isolation between the two data ports.

In another embodiment, the energy detect circuit is implemented as acontrol slew peak detector where the time constant of the circuit isdetermined by the selective arrangement of a series of resistors andcapacitors. In this manner, the relay switch is able to pass incomingsignals having a low pulse density, indicative of low energy level,while filtering out incoming signals that are data packets which have ahigh pulse density, indicative of high energy level.

The construction of the relay switch of the present invention will nowbe described. First, the connection of the relay switch in a PoweredDevice is described with reference to FIG. 2. FIG. 2 illustrates theincorporation of the relay switch of the present invention in a PoweredDevice coupled to a network device via a transmission cable according toone embodiment of the present invention. When the Powered Device isconfigured to respond to the signal loop back discovery method fordiscovering PoE capability, the relay switch of the present inventioncan facilitate such discovery while avoiding jamming of the networkdevice.

Referring to FIG. 2, a Powered Device 30 is connected to a networkdevice 1 through a transmission cable 3. Network device 1 may or may notbe a Power Sourcing Equipment and therefore may or may not be capable ofproviding power to Powered Device 30. In the present illustration, thenetwork configuration is an Ethernet and transmission cable 3 is assumedto be a twisted-wire-pairs cable transmitting differential signals.Furthermore, Powered Device 30 is assumed to be implementing the PoEtechnology by transmitting power through the cable on the data pairs. Atthe Powered Device, the twisted wire pairs of cable 3 are transformercoupled and power is received from the center tap of the isolationtransformer. The received power is coupled to an isolated DC-DCconverter 34 to transform the received voltage to a lower voltage moresuitable for the electronics in the Powered Device. A Vdd voltage (node501) for the Powered Device is thus derived. In the presentillustration, Powered Device 30 is also provided with the capability forreceiving power from an external power source (node 5), such as from abattery or from an outlet 7. The received power from the external powersource is also coupled to the electronics of the Powered Device as a Vddvoltage on node 501.

A relay switch 50 in accordance with the present invention has a firstdata port coupled to the receiving terminals (RX+/−) of Powered Device30 and a second data port coupled to the transmitting terminals (TX+/−)of Powered Device 30. In the present illustration, the incoming signals(or input signals) are differential signals and the receiving andtransmitting terminals are therefore differential signal terminals. Itis understood that in many network devices, the “transmitting” and“receiving” terminals are interchangeable and the designations thereofare symbolic only. Therefore, an important feature of the relay switchof the present invention is that the relay switch is fullybi-directional to enable signal conduction from either data port to theother data port, as will be described in more detail below.

Relay switch 50 is also coupled to receive the Vdd voltage (node 501) ofPowered Device 30 on a power supply terminal. As thus configured, relayswitch 50 receives the Vdd voltage, if any, applied to node 501 ofPowered Device 30, and also receives input signals on either of thefirst and second data ports. Relay switch 50 operates in response to theVdd voltage value on node 501 and the energy level of the input signalson either of the data ports to either open or close the one or moreconduction paths through the relay switch.

More specifically, the relay switch measures the energy of the incomingsignals and, in the absence of a power supply voltage, the operation ofthe relay switch is responsive to the energy level of the incomingsignals. The amount of energy of the incoming signals is proportional tothe density of the signal pulses. The control pulses transmitted fordiscovery purposes usually have limited data content and is thereforerepresented as a burst of data. The control pulses thus have associatedtherewith a low energy level as the control pulses have low pulsedensity. On the other hand, data signals are usually transmitted in theform of data packets that are transmitted more or less continuously. Thedata signals thus have a relatively high pulse density and thereforehave associated therewith a high energy level.

In operation, when Powered Device 30 receives a valid Vdd voltage onnode 501, either from the external power supply (node 5) or from theinternal power source (such as from DC/DC converter 34), relay switch 50detects the presence of the valid power supply voltage and opens theconduction paths to isolate the two data ports. Thus, whenever there isa valid Vdd voltage on node 501, relay switch 50 is open.

When relay switch 50 detects that Powered Device 30 has no power supplyvoltage, the relay switch is operative to allow only signals with lowenergy level to pass, thereby filtering out incoming signals of highenergy level. Specifically, relay switch 50 is open when a high energylevel is detected in the incoming signals and the relay switch is closedwhen a low energy level is detected in the incoming signals. Therefore,when Powered Device 30 has no power, the relay switch will operate topermit control pulses, if any, received on either of the data ports topass while filtering out data packets arriving on the data ports. Inthis manner, when the Powered Device has no power, the relay switch isoperated to return only control pulses to facilitate discovery andprevent the return of data signal packets which may cause jamming of thenetwork device that is transmitting the data signal packets to thePowered Device.

Thus, if network device 1 is a PSE, network device 1, initiating thediscovery process, will transmit control pulses having a unique sequenceon cable 3 to Powered Device 30. Relay switch 50, detecting no Vddvoltage provided to Powered Device 30, will measure the energy of theincoming control pulses. When relay switch 50 determines that the energyof the incoming control pulses is less than a predetermined threshold,relay switch 50 is closed and the control pulses pass through the relayswitch back on cable 3 to be returned to network device 1. When networkdevice 1 receives the returned control pulses, network device 1recognizes that Powered Device 30 is capable of receiving power andrequires power. Network device 1 will thereafter supply power to PoweredDevice 30 on cable 3. With the provision of power on cable 3, relayswitch 50 detects the presence of a valid Vdd voltage on node 501 andthe relay switch will open as the discovery process has been completed.

On the other hand, if network device 1 is not a PSE, network device 1may transmit data packets on cable 3 to Powered Device 30. If PoweredDevice 30 has no power, relay switch 50 will measure the energy of theincoming signals and will detect a high level of energy present. Relayswitch 50 in response will be open to prevent the data packets frombeing returned to network device 1. In this manner, jamming of networkdevice 1, which is not a PSE, is prevented. In the situation wherePowered Device 30 is connected to a network device that cannot providepower, as long as Powered Device 30 does not generate network traffic,the presence of Powered Device 30 does not disturb the network. Theresult is merely that Powered Device 30 cannot function unless power isprovided by another power source.

FIG. 2 is provided to illustrate one application where the relay switchof the present invention can be advantageously applied to a PoweredDevice to facilitate PoE discovery. The network configuration shown inFIG. 2 is illustrative only and one of ordinary skill in the art, uponbeing apprised of the present description, will appreciate that therelay switch of the present invention can be applied in a variety of PoEtopologies, such as the PoE topology where power is transmitted throughthe spare twisted wire pairs of the transmission cable. Furthermore, therelay switch of the present invention can be an integrated component ofthe Powered Device or an external component coupled to the PoweredDevice. The exact configuration and level of integration of the relayswitch is not critical to the practice of the present invention.

FIG. 3 is a block diagram of a relay switch according to one embodimentof the present invention. Referring to FIG. 3, relay switch 50 includesa main conduction switch circuit (main switch) 100, an energy detectcircuit 200 and a control signal generator 300. Relay switch furtherincludes a first data port (Port 1) coupled to receive or transmitdifferential signals on wires 11 and 12 and a second data port (Port 2)coupled to receive or transmit differential signals on wires 21 and 22.Relay switch 50 also includes a power supply terminal 60 coupled toreceive a power supply (Vdd) voltage which is usually the power supplyvoltage of the device to which the relay switch is coupled. For example,if relay switch 50 is coupled in Powered Device 30 of FIG. 2, then theVdd voltage on node 60 received by relay switch 50 will be the Vddvoltage on node 501 of Powered Device 30.

Main conduction switch circuit 100 is coupled to the first and seconddata ports to receive the incoming differential signals (input signals)and also coupled to receive a set of control signals Ctrl1, Ctrl2, Ctrl3and Ctrl4 from control signal generator 300. Under the direction of thecontrol signals, main switch 100 is either closed to electricallyconnect the first data port to the second data port or open toelectrically isolate the first data port from the second data port. Whenmain switch 100 is closed, data signals received on either data portwill be transmitted to the other data port. Main switch 100 is alsocoupled to the power supply terminal 60 to receive the power supply(Vdd) voltage. The Vdd voltage is used by main switch 100 to create ashunting path, as will be described in more detail below.

Energy detect circuit 200 is also coupled to the first and second dataports to receive the incoming differential signals. Energy detectcircuit 200 operates to measure the energy level of incoming signals onthe first and second data ports and generate a first energy detectsignal ED1 and a second energy detect signal ED2 as output signals.Specifically, the first energy detect signal ED1 is indicative of theenergy level of the incoming signals on the first data port while thesecond energy detect signal ED2 is indicative of the energy level of theincoming signals on the second data port. The first and second energydetect signals ED1 and ED2 are coupled to control signal generator 300to generate control signals Ctrl1 to Ctrl4. Energy detect circuit 200has to operate under conditions where there is no power supply voltage.Therefore, energy detect circuit 200 derives energy from the incomingsignal for its operation.

Control signal generator 300 is coupled to receive the Vdd voltagesupplied to power supply terminal 60 and also the first and secondenergy detect signals ED1 and ED2 from energy detect circuit 200.Control signal generator 300 generates control signals Ctrl1 to Ctrl4based on the state of the Vdd voltage and the energy detect signals ED1and ED2. Because relay switch 50 has to operate in conditions where nopower supply voltage is provided, control signal generator 300 derivesenergy for its operation from the input signals on the first data portand the second data port. Thus, control signal generator 300 is alsocoupled to the first data port and the second data port to receive theinput signals.

Control signals Ctrl1, Ctrl2, Ctrl3 and Ctrl4 generated by controlsignal generator 300 control the conduction state of main switch 100 ina manner described above. There is, control signals Ctrl1, Ctrl2, Ctrl3and Ctrl4 cause main switch 100 to be open when there is a valid Vddvoltage on power supply terminal 60 or when there is no Vdd voltage onthe power supply terminal but energy detect signals ED1 and ED2 indicatea high energy level in the incoming signals at either one of the dataports. On the other hand, control signals Ctrl1, Ctrl2, Ctrl3 and Ctrl4cause main switch 100 to be closed and conducting when there is no Vddvoltage on the power supply terminal but the energy detect signals ED1and ED2 indicate a low energy level in the input signals at either oneof the data ports.

The detail construction of each circuit block of relay switch 50 willnow be described. FIG. 4 is a circuit diagram of the main conductionswitch circuit of the relay switch according to one embodiment of thepresent invention. Referring to FIG. 4, main conduction switch circuit100 (or “main switch 100”) includes two circuit portions of identicalconstruction. The first circuit portion connects wire 11 of the firstdata port to wire 21 of the second data port. The second circuit portionconnects wire 12 of the first data port to wire 22 of the second dataport. Each circuit portion includes complementary MOSFET devices forforming a pair of conduction paths between the two wires so that signaltransmission between the two wires can be accomplished withoutappreciable attenuation.

In accordance with the present invention, each conduction path in mainswitch 100 is formed using a dual-switching-device structure. That is,each conduction path includes two serially connected switching devicesbetween the two wires. For instance, in the first circuit portion, NMOStransistor M21 and NMOS transistor M22 are connected in series betweenwire 11 and wire 21 to form a first conduction path. PMOS transistor M23and PMOS transistor M24 are connected in series between wire 11 and wire21 to form a second, complementary conduction path. In the secondcircuit portion, NMOS transistor M25 and NMOS transistor M26 areconnected in series between wire 12 and wire 22 to form a thirdconduction path. PMOS transistor M27 and PMOS transistor M28 areconnected in series between wire 12 and wire 22 to form a fourth,complementary conduction path.

The dual-switching device structure used for each conduction pathprovides improved isolation of the two ends of the conduction paths whenthe relay switch is open and the conduction paths are turned off. Thedual-switching device structure provides a marked improvement inisolation over switching circuit where a single transmission gate isused to connect the two wires at each end. Furthermore, at eachconduction path, a low-impedance shunting path is inserted between thetwo serially connected switching devices to shunt the conduction path toeither the Vdd voltage or the ground voltage. The low-impedance shuntingpath is effective to significantly reduce high frequency signaltransmission between the two ends of the conduction path when theswitching devices are open.

The conduction paths and shunting paths of main switch 100 arecontrolled by the control signals Ctrl1 to Ctrl4. Control signal Ctrl3(node 303) and control signal Ctrl4 (node 304) form a pair ofcomplementary control signals driving the first and second conductionpaths connecting wires 11 and 21. In the present embodiment, controlsignal Ctrl3 drives the gate terminal of NMOS transistors M21 and M22and PMOS transistor M29 while complementary control signal Ctrl4 drivesthe gate terminal of PMOS transistors M23 and M24 and NMS transistorM30. Thus, when control signals Ctrl3 and Ctrl4 are asserted to closethe relay switch, transistors M21, M22, M23 and M24 are turned on toclose the first and second conduction paths while transistors M29 andM30 are turned off. When control signals Ctrl3 and Ctrl4 are deassertedto open the relay switch, transistors M21, M22, M23 and M24 are turnedoff to open the first and second conduction paths while transistors M29and M30 are turned on to shunt the conduction paths to the Vdd voltageand the ground voltage respectively. In this manner, improved isolationof wires 11 and 21 when the relay switch is open is achieved.

The second circuit portion is controlled in a similar manner to thefirst circuit portion described above. Specifically, control signalsCtrl1 (node 301) and control signal Ctrl2 (node 302) form a pair ofcomplementary control signals driving the third and fourth conductionpaths connecting wires 12 and 22. In the present embodiment, controlsignal Ctrl1 drives the gate terminal of NMOS transistors M25 and M26and PMOS transistor M31 while complementary control signal Ctrl2 drivesthe gate terminal of PMOS transistors M27 and M28 and NMS transistorM32. Thus, when control signals Ctrl1 and Ctrl2 are asserted to closethe relay switch, transistors M25, M26, M27 and M28 are turned on toclose the third and fourth conduction paths while transistors M31 andM32 are turned off. When control signals Ctrl1 and Ctrl2 are deassertedto open the relay switch, transistors M25, M26, M27 and M28 are turnedoff to open the third and fourth conduction paths while transistors M31and M32 are turned on to shunt the conduction paths to the Vdd voltageand the ground voltage respectively. In this manner, improved isolationof wires 12 and 22 when the relay switch is open is achieved.

Turning now to the energy detect circuit of the relay switch of thepresent invention. FIG. 5 is a circuit diagram of the energy detectcircuit of the relay switch according to one embodiment of the presentinvention. Referring to FIG. 5, energy detect circuit 200 includes afirst detect circuit 200A for detecting the energy of input data signalson the first data port (Port 1) associated with wires 11 and 12 and asecond detect circuit 200B for detecting the energy of input datasignals on the second data port (Port 2) associated with wires 21 and22. The first detect circuit 200A and the second detect circuit 200Bhave identical construction and identical elements are given identicalreference numerals to simplify the discussion. In the followingdescription, the construction and operation of only the first detectcircuit 200A will be described in detail. It is understood that thefirst detect circuit 200A receives incoming signals from wires 11 and 12and generate an energy detect signal ED1 indicative of the energy levelof the incoming signals on wires 11 and 12. It is further understoodthat the second detect circuit 200B operates in the same manner toreceive incoming signals from wires 21 and 22 and generate an energydetect signal ED2 indicative of the energy level of the incoming signalson wires 21 and 22.

In the present description, the energy of a signal refers to the envelopof the signal pulses where a high pulse density in the signal pulses isassociated with a high energy level and a low pulse density in thesignal pulses is associated with a low energy level. The energy detectcircuit of the present invention measures the energy level of theincoming signals to determine if the signal pulses are high energy andtherefore are most likely data pulses or low energy and therefore aremost likely control pulses.

It is important to note that energy detect circuit 200 has to functionunder conditions where there is no power supply voltage and thereforeenergy detect circuit 200 is configured to obtain power from the inputsignals for its operation. In the present embodiment, a positive waveselector is used to accumulate power from the input signals. Referringto FIG. 5, first detect circuit 200A includes a pair of PMOS transistorsM33 and M34 connected to the differential input signals of wires 11 and12 in opposite polarities to form the positive wave selector.Specifically, PMOS transistor M33 has a drain terminal coupled to wire11 and a gate terminal coupled to wire 12 while PMOS transistor M34 hasa drain terminal coupled to wire 12 and a gate terminal coupled to wire11. Because the signals on wires 11 and 12 are differential signals, thealternate coupling of the differential signals to transistors M33 andM34 results in the passing of only positive waves on either wiresthrough the transistors.

In operation, when the signal on wire 11 is high and the signal on wire12 is accordingly low, transistor M33 is turned on while transistor M34is turned off. The positive wave on wire 11 thus passes throughtransistor M33 while the negative wave on wire 12 is blocked.Alternately, when the signal on wire 11 is low and the signal on wire 12is accordingly high, transistor M33 is turned off while transistor M34is turned on. The positive wave on wire 12 thus passes throughtransistor M34 while the negative wave on wire 11 is blocked. Thus, oneof the transistors M33 and M34 forming the positive wave selector willpass a positive wave and one of the transistors will guarantee to turnon as the two signals on wires 11 and 12 are differential signals. Animportant advantage of the positive wave selector used in the energydetect circuit of the present invention is that no voltage drop isincurred across the positive wave selector. This is a significantimprovement over conventional positive wave selectors where a dioderectifier is used to accomplish full wave rectification and appreciablevoltage drop is induced across the diode rectifier.

To realize the energy detect function, first detect circuit 200Aincludes a resistor R31 and a resistor R33 connected in series betweenthe source terminal of transistor M33 and the ground node. First detectcircuit 200A further includes a resistor R32 and a capacitor C31connected in series between the source terminal of transistor M34 andthe ground node. The common node between resistors R31 and R33 and thecommon node between resistor R32 and capacitor C31 are shorted togetherto form a single common node 250. Common node 250 is connected to aresistor R34 to generate the energy detect signal ED1 at node 201.

The operation of the energy detect circuitry of first detect circuit200A is as follows. Resistors R31 and R32 convert the voltage signal atthe respective source terminals of transistors M33 and M34 into acurrent where the current is coupled to charge capacitor C31. Theresistance of resistors R31 and R32 is selected so that only a portionof the current is permitted to pass to charge up capacitor C31.Resistors R31 and R32 also operate to isolate wires 11 and 12 from eachother. Resistor R33, on the other hand, is connected in parallel withcapacitor C31 and functions as a discharging path for the capacitor.While capacitor C31 is being charged up, resistor R33 continuouslydischarges the capacitor. The charging of capacitor C31 is therefore afunction of the pulse density of the input signals on wires 11 and 12.The resistance of resistors R31 to R33 and the capacitance of capacitorC31 are selected to realize a desired time constant where input signalshaving a high pulse density are able to charge up capacitor C31.

Specifically, as positive waves are received by transistors M33 and M34and passed through resistors R31 and R32 to charge capacitor C31,resistor R33 discharges the capacitor. Thus, when the positive waves atwires 11 and 12 have a high pulse density, capacitor C31 charges upfaster than resistor R33 can discharge it. Therefore, the voltage atcommon node 250 increases. On the other hand, when the positive waves atwires 11 and 12 have a low pulse density, resistor R33 dischargescapacitor C31 faster than charge can be accumulated at the capacitor andcapacitor C31 cannot be charged up. The voltage at common node 250therefore decreases towards the ground potential.

Resistor R34, coupled between common node 250 and output node 201, isused to adjust the time constant of first detect circuit 200A as givenby resistors R31 to R33 and capacitor C31. Resistor R34 also providessome degree of isolation for the output node 201. Resistor R34 isoptional and may be omitted in other embodiments of the presentinvention.

Through the charging and discharging of capacitor C31, a voltageindicative of the pulse density of the input signals on wires 11 and 12is generated at node 250 and attenuated by resistor R34 to be used asthe first energy detect signal ED1 on node 201. Energy detect signal ED1is therefore a signal indicative of the energy level of the inputsignals on wires 11 and 12. Similarly, at second detect circuit 200B,energy detect signal ED2 is generated as a signal indicative of theenergy level of the input signals on wires 21 and 22.

Turning now to the control signal generator of the relay switch of thepresent invention. FIG. 6 is a circuit diagram of the control signalgenerator of the relay switch according to one embodiment of the presentinvention. Referring to FIG. 6, control signal generator 300 includes afirst generator circuit 310 and a second generator circuit 320 ofidentical construction and operating on different polarities of theinput signals on wires 11, 12, 21 and 22. Each generator circuitincludes input ports A, B, C and D for receiving the differential inputsignals on wires 11, 12, 21 and 22. Each generator circuit also receivesthe Vdd voltage (node 60).

First generator circuit 310 has wire 11 coupled to port A, wire 12coupled to port B, wire 21 coupled to port C and wire 22 coupled to portD. First generator circuit 310 generates a pair of complementary controlsignals Ctrl1P and Ctrl2P on nodes 331 and 332 as output signals. On theother hand, second generator circuit 320 has wire 11 coupled to port B,wire 12 coupled to port A, wire 21 coupled to port D and wire 22 coupledto port C. Second generator circuit 320 generates the pair ofcomplementary control signals Ctrl3P and Ctrl4P on nodes 333 and 334 asoutput signals. Control signals Ctrl1P, Ctrl2P, Ctrl3P and Ctrl4P areprecursor signals to the final control signals Ctrl1, Ctrl2, Ctrl3 andCtrl4 generated by control signal generator 300, as will be explained inmore detail below.

Control signal generator 300 further includes a first energy detectcontrol circuit (Energy Detect Control 1) 360 and a second energy detectcontrol circuit (Energy Detect Control 2) 362 coupled to operate on theprecursor control signals Ctrl1P, Ctrl2P, Ctrl3P and Ctrl4P. Firstenergy detect control circuit 360 drives the precursor control signalsin response to the first energy detect signal ED1 while second energydetect control circuit 362 drives the precursor control signals inresponse to the second energy detector signal ED2. Finally, in controlsignal generator 300, precursor control signals Ctrl1P, Ctrl2P, Ctrl3Pand Ctrl4P on nodes 331 to 334 are coupled to respective first andsecond latch circuits 620, 640 to generate the final control signalsCtrl1, Ctrl2, Ctrl3 and Ctrl4 on nodes 301 to 304. First and secondlatch circuits 620, 640 each receives the input signals on wires 11, 12,21 and 22 from the first and second data ports and the Vdd voltage tofacilitate its operation. Each of first and second latch circuits 620,640 operates through feedback to reinforce the signal levels of theprecursor control signals to generate final control signals Ctrl1,Ctrl2, Ctrl3 and Ctrl4 having the desired voltage levels.

The detail construction of the circuit blocks of control signalgenerator 300 will now be described. First and second generator circuits310 and 320 have identical constructions and the detail circuit diagramsare illustrated in FIGS. 7-9. Identical circuit elements are givenidentical reference numerals to simplify the discussion. In thefollowing description, the construction and operation of only the firstgenerator circuit 310 for generating precursor control signals Ctrl1Pand Ctrl2P will be described in detail. It is understood that secondgenerator circuit 320 is constructed and operated in the same manner asfirst generator circuit 310 to generate the corresponding precursorcontrol signals Ctrl3P and Ctrl4P.

FIG. 7 is a circuit diagram of a Vdd level generator circuitincorporated in control signal generator 300 according to one embodimentof the present invention. Referring to FIG. 7, Vdd level generatorcircuit 350 is coupled to receive the Vdd voltage (node 60) of the relayswitch and generate a VDD_L signal and a VDD_H signal in response. Boththe VDD_H and the VDD_L signals have logical low values when the Vddvoltage is less than a predetermined Vdd voltage threshold level. TheVDD_H signal has a logical high value while the VDD_L signal has alogical low value when the Vdd voltage is greater than the predeterminedVdd voltage threshold level. The voltage difference between the VDD_Hand VDD_L signals are used to drive Vdd control circuits in first andsecond generator circuits 310 and 320, as will be explained in moredetail below. In control signal generator 300, only one instance of Vddlevel generator circuit 350 is required to generate the VDD_L and VDD_Hsignals for both first and second generator circuits 310 and 320.However, in other embodiments, separate Vdd level generator circuits canbe provided for each of first and second generator circuits 310 and 320.

In Vdd level generator circuit 350, the Vdd voltage (node 60) is coupledto the drain terminal of a PMOS transistor M59. The gate terminal oftransistor M59 is connected to the ground potential. The source terminal(node 63) of transistor M59 is the VDD_H signal. A resistor R59 isconnected between the source terminal of transistor M59 and the groundpotential. The VDD_H signal on node 63 is coupled to an inverter formedby serially connected PMOS transistor M57 and NMOS transistor M58. TheVDD_L signal is generated at the output node 62 of the inverter.

Transistor M59 and resistor R59 in Vdd level generator circuit 350 setthe threshold level for indicating a valid Vdd voltage. Morespecifically, the transistor threshold voltage V_(T) of transistor M59and the voltage divider ratio established by transistor M59 and resistorR59 set the predetermined Vdd voltage threshold level for indicating avalid Vdd voltage. One of ordinary skill in the art would appreciatethat other circuit arrangement can be used to select a differentthreshold level for indicating a valid Vdd voltage.

In operation, when the Vdd voltage at node 60 is less than thetransistor threshold voltage (V_(T)) of transistor M59, transistor M59is not turned on and the VDD_H signal is pulled low by resistor R59. TheVDD_L signal also remains at a logical low level since there is nosufficient Vdd voltage at the inverter circuit. When both the VDD_H andthe VDD_L signals are at the logical “low” levels, the Vdd levelgenerator circuit will not activate the subsequent circuitry in thefirst and second generator circuits 310 and 320, as will be explained inmore detail below.

When the Vdd voltage at node 60 is equal to or greater than thetransistor threshold voltage of transistor M59, transistor M59 is turnedon and starts to conduct. Thus, transistor M59 and resistor R59 form avoltage divider for the Vdd voltage. The voltage level of the VDD_Hsignal is thus a fraction of the Vdd voltage. When the VDD_H signal hasa voltage level sufficient to trigger the inverter formed by transistorsM57 and M58, the inverter is activated and the VDD_L signal is forced tothe ground potential through transistor M58. The voltage differencebetween the VDD_H signal and the VDD_L will be used by Vdd controlcircuits in the first and second generator circuits to generate controlsignals in response to the Vdd voltage level, as will be explained inmore detail below.

FIG. 8 is a circuit diagram of first generator circuit 310 of controlsignal generator 300 according to one embodiment of the presentinvention. FIG. 9 is a circuit diagram of second generator circuit 320of control signal generator 300 according to one embodiment of thepresent invention. As can be observed from FIGS. 8 and 9, theconstruction of the first and second generator circuits is identical andthe circuits differ only with respect to the arrangement of inputsignals connected to ports A, B, C and D and the control signalsgenerated there from. Therefore, only first generator circuit 310 willbe described in detail below and second generator circuit 320 will onlybe described in brief. It is understood that second generator circuit320 is constructed and operates in the same manner as first generatorcircuit 310.

Referring to FIG. 8, first generator circuit 310 includes a firstcircuit portion for generating precursor control signal Ctrl1P and asecond circuit portion for generating the complementary precursorcontrol signal Ctrl2P. Because control signals Ctrl1P and Ctrl2P arecomplementary, first generator circuit 310 operates to generate activelyonly one of the two control signals while the other complementarycontrol signal is generated passively using a latch circuit, as will beexplained in more detail below.

Referring to FIG. 9, second generator circuit 320 includes a firstcircuit portion for generating precursor control signal Ctrl3P and asecond circuit portion for generating the complementary precursorcontrol signal Ctrl4P. Again, because control signals Ctrl3P and Ctrl4Pare complementary, second generator circuit 320 operates to generateactively only one of the two control signals while the othercomplementary control signal is generated passively using a latchcircuit.

Because control signal generator 300 has to operate under conditions ofno power supply, the control signal generator includes positive waveselectors similar to that incorporated in energy detect circuit 200 toaccumulate power for its operation. Returning to FIG. 8, in the firstcircuit portion of first generator circuit 310, PMOS transistor M51 andPMOS transistor M52 form a positive wave selector for accumulatingenergy from respective wires 11 and 12, and wires 21 and 22 in a firstpolarity. Specifically, PMOS transistor M51 has a drain terminal coupledto wire 11 and a gate terminal coupled to wire 12 while PMOS transistorM52 has a drain terminal coupled to wire 21 and a gate terminal coupledto wire 22. As thus configured, PMOS transistors M51 and M52 willaccumulate positive-going waves appearing on wires 11 and 21. Thevoltages appearing on the source terminals of transistors M51 and M52are converted to currents by resistors R1 and supplied to node 331.

In the second circuit portion of first generator circuit 310, NMOStransistor M54 and NMOS transistor M55 form a positive wave selector foraccumulating energy from respective wires 11 and 12, and 21 and 22 in asecond, opposite polarity. Specifically, NMOS transistor M54 has a drainterminal coupled to wire 11 and a gate terminal coupled to wire 12 whileNMOS transistor M55 has a drain terminal coupled to wire 21 and a gateterminal coupled to wire 22. As thus configured, NMOS transistors M54and M55 will accumulate negative-going waves appearing on wires 11 and21. The voltages appearing on the source terminals of transistors M54and M55 are converted to currents by resistors R3 and supplied to node332.

The positive wave selectors formed by transistors M51, M52, M54 and M55accumulate energy from the incoming signals to support the operation offirst generator circuit 310. Similar positive wave selectors areincluded in second generator circuit 320 to accumulate energy from theincoming signals to support the operation of the second generatorcircuit.

Nodes 331 and 332 are the main control nodes of the first generatorcircuit 310. The main control nodes driven in a manner to respond to theVdd voltage level. In the present embodiment, main control node 331 inthe first circuit portion is driven by a Vdd control circuit 370 whilemain control node 332 in the second circuit portion is driven by a Vddcontrol circuit 372. As shown in FIG. 6 and will be explained in moredetail below, main control nodes 331 and 332 are also driven by firstenergy detect control circuit 360 and second energy detect controlcircuit 362 to cause the precursor control signals to respond to theenergy detect signals.

In the first circuit portion of first generator circuit 310, Vdd controlcircuit 370 includes an NMOS transistor M53 having its source terminalcoupled to the VDD_L signal and its gate terminal coupled to the VDD_Hsignal. Thus, when the VDD_H voltage is greater than the VDD_L voltageby the transistor threshold voltage (V_(T)) of NMOS transistor M53,transistor M53 is turned on and the logical low level of the VDD_Lsignal will pass through transistor M53 to a resistor R2 coupled to thedrain terminal of transistor M53. Resistor R2 turns the voltage value ofthe VDD_L signal into a current for driving main control node 331. Maincontrol node 331, which provides the precursor control signal Ctrl1P, isthus driven to the ground potential.

In the second circuit portion, Vdd control circuit 372 includes a PMOStransistor M56 having its source terminal coupled to the VDD_H signaland its gate terminal coupled to the VDD_L signal. Thus, when the VDD_Hvoltage is greater than the VDD_L voltage by the transistor thresholdvoltage (V_(T)) of PMOS transistor M56, transistor M56 is turned on andthe logical high level of the VDD_H signal will pass through transistorM56 to a resistor R4 coupled to the drain terminal of transistor M56.Resistor R4 turns the voltage value of the VDD_H signal into a currentfor driving main control node 332. Main control node 332, which providesthe precursor control signal Ctrl2P, is thus driven to a logical highlevel.

Referring to FIG. 9, Vdd control circuits 374 and 376 operate in thesame manner as described above to drive main control node 333 and 334for providing the precursor control signals Ctrl3P and Ctrl4P.Furthermore, precursor control signals Ctrl1P, Ctrl2P, Ctrl3P and Ctrl4Pare in fact identical to the final control signals Ctrl1, Ctrl2, Ctrl3and Ctrl4 with the final control signals having their voltage levelsreinforced by the latch circuits.

By reference back to FIG. 4, when control signals Ctrl1 and Ctrl2 havingrespective low and high logical levels in response to a valid Vddvoltage are used to drive the conduction paths connecting wires 12 and22, the conduction paths of transistors M25-M28 will be open and notransmission between wires 12 and 22 occurs. Control signals Ctrl3 andCtrl4, generated by second generator circuit 320 in the same manner asfirst generator circuit 310, will also operate to open up the conductionpaths of transistors M21 to M24 in response to a valid Vdd signal.Therefore, in response to a valid Vdd voltage, main switch 100 is open.Furthermore, the state of the control signals Ctrl1 to Ctrl4 will causetransistors M29 to M32 forming the shunting paths to turn on to shuntthe respective conduction path to the Vdd voltage or to ground.Isolation of the first and second data ports when main switch 100 isopen is thus achieved.

Returning to FIG. 8, Vdd control circuits 370 and 372 provide anoverriding control of the main control nodes 331 and 332. That is, whena valid Vdd voltage is detected, Vdd control circuits 370 and 372 willdrive main control nodes 331 and 332 to the respective state to causethe relay switch to open, regardless of the control provided by othercontrol circuits, such as the energy detect control circuits 306 and 362(FIG. 6). When there is no Vdd voltage or the Vdd voltage is below thevalid level, Vdd control circuits 370 and 372 will be disabled and willnot drive the respective main control nodes 331 and 332. Instead, thestate of the precursor control signals will then be determined by energydetect control circuits 360 and 362.

Returning to FIG. 6, the precursor control signals Ctrl1P, Ctrl2P,Ctrl3P and Ctrl4P on nodes 331 to 334 generated by first and secondgenerator circuits 310, 320 are further driven by first and secondenergy detect control circuits 360 and 362. In operation, energy detectcircuit 360, responsive to first energy detect signal ED1, will drivecontrol signals Ctrl1P and Ctrl3P (main control nodes 331 and 333) to alogical low level and control signals Ctrl2P and Ctrl4P (main controlnodes 332 and 334) to a logical high level when first energy detectsignal ED1 indicates the detection of a high energy level on wires 11and 12. On the other hand, energy detect circuit 362, responsive tosecond energy detect signal ED2, will drive control signals Ctrl1P andCtrl3P (main control nodes 331 and 333) to a logical low level andcontrol signals Ctrl2P and Ctrl4P (main control nodes 332 and 334) to alogical high level when second energy detect signal ED2 indicatesdetection of a high energy level on wires 21 and 22.

Thus, when there is no Vdd voltage and a high energy level is detectedin the incoming signals on either data port of the relay switch, mainswitch 100 is open and no conduction occurs between wires 11, 12 andwires 21 and 22. Note that even for incoming signals with high pulsedensity, it will take a certain amount of time for the energy detectcapacitor C31 (FIG. 5) to be charged up to indicate a high energy level.Therefore, the relay switch will pass a portion of the incoming datasignals before the relay switch is open. Because any data signals thatgot passed through are merely partial data packet, the passed-throughdata signals are treated as invalid data and will be ignored by thenetwork device receiving them and will not be retransmitted.

On the other hand, when there is no Vdd voltage and a low energy levelis detected in the incoming signals on either data port of the relayswitch, energy detect control circuits 360 and 362 will not drive any ofthe main control nodes. Instead, the state of the precursor controlsignals will be determined by the data signals on wires 11, 12, 21 and22, as will be described in more detail below.

FIG. 10 is a circuit diagram of energy detect control circuit 360 ofFIG. 6 according to one embodiment of the present invention. FIG. 11 isa circuit diagram of energy detect control circuit 362 of FIG. 6according to one embodiment of the present invention. Note that circuit360 of FIG. 10 and circuit 362 of FIG. 11 have identical constructionsexcept for the input signals (ED1 or ED2) coupled thereto and the outputsignals generated there from. Description of only energy detect controlcircuit 360 in FIG. 10 will be provided below and one of ordinary skillin the art would appreciate that energy detect control circuit 362 isconstructed and operated in the same manner as energy detect controlcircuit 360.

Referring to FIG. 10, energy detect control circuit 360 receives firstenergy detect signal ED1 as an input signal (node 201). An energy levelgenerator circuit 365 is coupled to receive first energy detect signalED1 and generates an ED1_L signal and an ED1_H signal in response. Itcan be observed that energy level generator circuit 365 is constructedin the same manner as Vdd level generator circuit 350 of FIG. 7.Specifically, circuit 365 includes a PMOS transistor M37 and a resistorR27 connected in series between node 201 receiving the ED1 signal andthe ground potential. The common node 203 between transistor M37 andresistor R37 is the ED1_H signal. The gate terminal of transistor M37 isconnected to the ground potential. The ED1_H signal on node 203 iscoupled to an inverter formed by serially connected PMOS transistor M35and NMOS transistor M36. The ED1_L signal is generated at the outputnode 202 of the inverter.

Transistor M37 and resistor R37 in energy level generator circuit 365set the energy threshold level for indicating a high energy level. Morespecifically, the transistor threshold voltage V_(T) of transistor M37and the voltage divider ratio established by transistor M37 and resistorR37 set the predetermined energy threshold level for indicating a highenergy level. One of ordinary skill in the art would appreciate thatother circuit arrangement can be used to select a different energythreshold level for indicating a high energy level.

The operation of energy level generator circuit 365 is identical to Vddlevel generator circuit 350. Basically, ED1_H signal has a logical highvalue and ED1_L signal has a logical low value when signal ED1 is at alogical high level. Alternately, both ED1_H and ED1_L signals havelogical low values when signal ED1 is at a logical low level.

In energy detect control circuit 360, the ED1_L and ED1_H signals arecoupled to separate positive wave selector circuits to generate thecorresponding control signals for driving the main control nodes 331-334of the control signal generator circuits. First, PMOS transistors M38and M39 are connected in parallel to receive the ED1_H signal at thedrain terminals and the ED1_L signal at the gate terminals. Thus, whenfirst energy detect signal ED1 is at a logical high, indicative of ahigh energy level, transistors M38 and M39 are turned on to pass theED1_H signal to respective resistors R5. Precursor control signalsCtrl2P and Ctrl4P are thus driven to a logical high level. At the sametime, NMOS transistors M40 and M41 are connected in parallel to receivethe ED1_L signal at the drain terminals and the ED1_H signal at the gateterminals. Thus, when first energy detect signal ED1 is at a logicalhigh, indicative of a high energy level, transistors M40 and M41 areturned on to pass the ED1_L signal to respective resistors R6. Precursorcontrol signals Ctrl1P and Ctrl3P are thus driven to a logical lowlevel.

As thus operated, energy detect control circuit 360 drives precursorcontrol signals Ctrl1P and Ctrl3P to a logical low level and precursorcontrol signals Ctrl2P and Ctrl4P to a logical high level when the firstenergy detect signal ED1 indicates a high energy level on wires 11 and12. The final control signals Ctrl1, Ctrl2, Ctrl3 and Ctrl4 will havethe same logical states as the precursor control signals. By referenceto FIG. 4, the logical states of the control signals cause theconduction paths of the main switch to open. Therefore, the relay switchof the present invention is open and isolate the first and second dataports when the Vdd voltage is absent and a high energy level is detectedin the incoming signals. Energy detect control circuit 360 is disabledwhen energy detect signal ED1 is at a logical low level indicative of alow energy level, thereby leaving the main control nodes 331-334undriven by the energy detect control circuit.

Energy detect control circuit 362 of FIG. 11 operates in the same manneras described above to drive precursor control signals Ctrl1P to Ctrl4Pin response to second energy detect signal ED2 indicative of the energylevel of the incoming signals on wires 21 and 22.

Referring to FIGS. 4 and 6-11, the control signal generator 300 forcescontrol signals Ctrl1 and Ctrl3 to a logical low state and controlsignals Ctrl2 and Ctrl4 to a logical high state to open the relay switchin response to a valid Vdd voltage level and to a high energy leveldetected in the incoming signals. In the absence of a valid Vdd voltageand in the absence of a high detected energy level, the Vdd controlcircuits and the energy detect control circuits are no longer drivingthe main control nodes 331-334 of control signal generator 300. Instead,control signal generator 300 generates the final control signals Ctrl1to Ctrl4 using the positive wave selectors of transistors M51, M52, M54,M55 in each of first generator circuit 310 and second generator circuit320.

Referring first to FIGS. 6 and 8, when main control nodes 331 and 332are not driven by any of the Vdd control circuits 370, 372, energydetect control circuits 360 and 362, the main control nodes are drivenentirely by the energy accumulated through transistors M51, M52, M54 andM55. First, assume that incoming signals are arriving on wires 11 and 12associated with the first data port. When the signal on wire 11 has alogical high value, the signal on wire 12, being a differential signal,has a logical low value. Thus, PMOS transistor M51 is turned on and thelogical high value on wire 11 passes through resistor R1 to drive maincontrol node 331 (Ctrl1P) to a logical high level. Meanwhile, NMOStransistor M54 is turned off and main control node 332 (Ctrl2P) is notdriven. Now, when the signal on wire 11 has a logical low value, thesignal on wire 12, being a differential signal, has a logical highvalue. Thus, PMOS transistor M54 is turned on and the logical low valueon wire 11 passes through resistor R3 to drive main control node 332(Ctrl2P) to a logical low level. Meanwhile, NMOS transistor M51 isturned off and main control node 321 (Ctrl1P) is not driven.

As thus configured, when control signals Ctrl1P and Ctrl2P are drivenentirely by the incoming pulses from the data ports, only one of the twoprecursor control signals will be actively generated. It is alsoapparent that the same result is obtained in first generator circuit 310when incoming signals arrive on wires 21 and 22 of the second data port.Furthermore, the same analysis applies to second generator circuit 320(FIG. 9) where only one of precursor control signals Ctrl3P and Ctrl4Pwill be actively generated when the control signals are driven entirelyfrom the data ports.

However, because control signals Ctrl1P and Ctrl2P belong to acomplementary pair and control signals Ctrl3P and Ctrl4P belong to acomplementary pair, when one control signal in the pair is activelygenerated, the other complementary control signal can be generatedpassively from the complementary control signal through feedback. Inaccordance with the present embodiment of the present invention, latchcircuits 620, 640 (FIG. 6) are used to force one control signal of thepair to a first logical state when the other control signal of the pairis actively forced to a second, complementary logical state by the firstand second generator circuits. In this manner, all the conduction pathsof the main conduction switch circuit can be turned on properly toprovide signal transmission without attenuation.

As described above, each of first and second latch circuits 620, 640operates through feedback to reinforce the signal levels of theprecursor control signals to generate final control signals Ctrl1,Ctrl2, Ctrl3 and Ctrl4 having the desired voltage levels. FIG. 12 is acircuit diagram of first latch circuit 620 which can be usedincorporated in control signal generator 300 of FIG. 6 according to oneembodiment of the present invention. FIG. 13 is a circuit diagram ofsecond latch circuit 640 which can be incorporated in the control signalgenerator 300 of FIG. 6 according to one embodiment of the presentinvention. Referring to FIGS. 12 and 13, first latch circuit 620operates through feedback to generate a final control signal in thecomplementary pair of control signals Ctrl1 and Ctrl2 while second latchcircuit 640 operate through feedback to generate a final control signalin the complementary pair of control signals Ctrl3 and Ctrl4. Theconstruction and operation of first and second latch circuits 620 and640 are identical except for the input and output signals. Therefore,the following description refers only to the first latch circuit 620.

Referring to FIG. 12, latch circuit 620 includes a PMOS transistor M3connected between wire 22 and a node E, a PMOS transistor M4 connectedbetween wire 21 and node E, a PMOS transistor M7 connected between theVdd voltage (node 60) and node E, and an NMOS transistor M1 connectedbetween node E and the ground potential. The gate terminals oftransistors M1, M3, M4 and M7 are all coupled to receive precursorcontrol signal Ctrl1P through a resistor R8. Latch circuit 620 furtherincludes a PMOS transistor M5 connected between wire 11 and a node F, aPMOS transistor M6 connected between wire 12 and node F, a PMOStransistor M8 connected between the Vdd voltage and node F, and an NMOStransistor M2 connected between node F and the ground potential. Thegate terminals of transistors M2, M5, M6 and M8 are all coupled toreceive precursor control signal Ctrl2P through a resistor R8. Node E(also node 302) is cross-coupled to connect through resistor R9 to thegate terminals of transistors M2, M5, M6 and M8 while node F (also node301) is cross-coupled to connect through resistor R9 to the gateterminals of transistors M1, M3, M4 and M7.

In operation, assume that wire 11 is at a logical high level and wire 12is therefore at a logical low level, control signal Ctrl1P is driven toa logical high value by first generator circuit 310 (FIG. 8). Thelogical high value of control signal Ctrl1P turns on transistor M1 whichdrives node E to the ground potential. Because node E is control signalCtrl2, final control signal Ctrl2 is thereby being driven to a logicallow level, which is the complementary state of control signal Ctrol1P.Meanwhile, the logical low value at control signal Ctrl2 is coupledthrough resistor R9 to turns on transistor M5. Control signal Ctrl1 atnode F is therefore driven to the logical high value of wire 11. Thelogical high value of wire 11 is thus coupled back to reinforce thelogical high value of final control signal Ctrl1. As thus operated,final control signal Ctrl1 have the same logical value as precursorcontrol signal Ctrl1P but with boosted signal level or boosted voltagevalue. Final control signal Ctrl2 is forced to a logical state oppositeto the final control signal Ctrl1 and is also driven to a boosted signallevel or boosted voltage value.

One of ordinary skill in the art, upon inspection of first latch circuit620 (FIG. 12) and second latch circuit 640 (FIG. 13), would appreciatethat the latch circuits operate in the manner describe above to drivethe one control signal of a complementary pair to a complementary statewhen the other control signal is actively generated by the first andsecond generator circuits. The advantage of first and second latchcircuits 620, 640 is that the final control signals are driven veryclose to the data signal values on the data ports. As can be observedfrom the circuit diagrams of FIGS. 12 and 13, the final control signalsCtrl1 to Ctrl4 on nodes 301 to 304 are only one transistor away from thewires of the data ports and the Vdd voltage. The final control signalsCtrl1 to Ctrl4 can thus be driven very close to the voltage levels atthe data ports or the Vdd node. The resulting control signals Ctrl1 toCtrl4 can thus have stronger signal strength.

In first and second latch circuits 620, 640, a resistor R9 is insertedbetween the final control signal node (E or F) and the precursor controlsignal (through resistor R8). Resistor R9 is included to allow certainsignal to assert influence and control over a node based on the voltagedivider principle. For instance, in first and second latch circuits 620,640, resistor R9 is incorporated to weaken the control of precursorcontrol signals Ctrl1P to Ctrl4P over respective nodes 301 to 304 andthereby allowing the final control signals Ctrl1 to Ctrl4 on nodes 301to 304 to be regenerated by the latch circuits. When final controlsignals Ctrl1 to Ctrl4 are thus regenerated, the control signals willhave strong voltage levels without appreciable attenuation.

In the above-described embodiment, control signal generator 300 includesseparate Vdd control circuit and energy detect control circuits.According to an alternate embodiment of the present invention, the Vddcontrol circuit is merged into each of the first and second energydetect control circuits to simplify the control of the main controlnodes. FIG. 14 is a circuit diagram of a first Vdd/energy detect controlcircuit according to one embodiment of the present invention. FIG. 15 isa circuit diagram of a second Vdd/energy detect control circuitaccording to one embodiment of the present invention. By using the firstand second Vdd/energy detect control circuits, Vdd control circuits 370,372, 374 and 376 (FIGS. 8 and 9) in the first and second generatorcircuits 310, 320 can be eliminated. Instead, first and second energydetect control circuits 360, 362 (FIG. 6) can be modified to includefirst and second Vdd/energy detect control circuits of FIGS. 14 and 15,as will be described in more detail below. The operation of controlsignal generator 300 remains the same.

Referring to FIG. 14, first Vdd/energy detect control circuit 420combines the function of the Vdd level generator circuit 350, Vddcontrol circuit 370, 372 and energy level generator circuit 365 of theenergy detect control circuit 360. That is, first Vdd/energy detectcontrol circuit 420 replaces the Vdd level generator circuit (350), theVdd control circuit (370, 372) and the energy level generator circuit365 of first energy detect control circuit 360. First Vdd/energy detectcontrol circuit 420 receives the Vdd voltage and the first energy detectsignal ED1 to generate a ED1_L(VDD_L) signal and a ED1_H(VDD_H) signalon respective nodes 202 and 203. The ED1_L(VDD_L) signal and theED1_H(VDD_H) signal are coupled to the remaining circuitry of firstenergy detect control circuit 360 (FIG. 10) to drive the main controlnodes 331 to 334.

Similar operation scheme applies to second Vdd/energy detect controlcircuit 430 of FIG. 15. Second Vdd/energy detect control circuit 430replaces the Vdd level generator circuit (350), the Vdd control circuit(374, 376) and the energy level generator circuit 367 of second energydetect control circuit 362. Second Vdd/energy detect control circuit 430generates a ED2_L(VDD_L) signal and a ED2_H(VDD_H) signal on respectivenodes 212 and 213. The ED2_L(VDD_L) signal and the ED2_H(VDD_H) signalare coupled to the remaining circuitry of energy detect control circuit362 (FIG. 11) to drive the main control nodes 331 to 334.

In first Vdd/energy detect control circuit 420, the Vdd voltage iscoupled to the anode of a diode D1 where the cathode is coupled to anode 422. First energy detect signal ED1 is coupled through a resistorR7 also to node 422. Thus, a high level on either the Vdd voltage or theenergy detect signal ED1 will cause node 422 to be at a logical high. aPMOS transistor M44, having its gate terminal coupled to the groundpotential, is connected between node 422 and node 203 providing theED1_H(VDD_H) signal. A resistor R44 is connected between node 203 andthe ground potential to establish a voltage divider function withtransistor M44 for setting the threshold level of the Vdd voltage andthe energy level. The ED1_H(VDD_H) signal is coupled to an inverterformed by a PMOS transistor M42 and an NMOS transistor M43 to generatethe ED1_L(VDD_L) signals on node 202. Diode D1 is included to preventthe Vdd node from being driven by the energy detect signal.

In operation, when node 422 is at a logical high level, signalED1_H(VDD_H) is driven high while signal ED1_L(VDD_L) is driven low. Thesignals ED1_H(VDD_H) and ED1_L(VDD_L) are then coupled to transistorsM38, M39, M40 and M41 of energy detect control circuit 360 to drive themain control nodes 331-334. The voltage difference between theED1_H(VDD_H) signal and the ED1_L(VDD_L) will allow transistors M38 toM41 to drive the precursor control signals so that the conduction pathsof the relay switch are turned off, isolating the first and second dataports. On the other hand, when node 422 is at a logical low level,signals ED1_H(VDD_H) and ED1_L(VDD_L) are both driven low. Energy detectcontrol circuit 360 no longer drives the main control nodes.

Second Vdd/energy detect control circuit 430 (FIG. 15) operates in thesame manner as first Vdd/energy detect control circuit 420 in responseto second energy detect signal ED2.

The relay switch of the present invention provides many advantages overconventional relay switch. First, the relay switch of the presentinvention incorporates energy detection to enhance the relay operation.In essence, the relay switch becomes a “smart” relay when the openingand closing of the switch is merely controlled by the Vdd voltage.Second, the relay switch of the present invention can be advantageouslyapplied in PoE applications where the signal loop back discovery methodis used. The relay switch of the present invention allows control pulsesto be looped back while preventing data packets to be returned, therebyavoiding jamming when the network device is not a PSE. Thus, the relayswitch can be incorporated for use with a Powered Device while avoidingthe need for a complicated filter to filter out the loop back signal.

Moreover, the relay switch of the present invention also hasapplications in the area of Voice over Internet Protocol (VoIP) for 911dialing. VoIP allows the user to make inexpensive telephone calls overthe Internet. But one pitfall of VoIP telephone is that, just likemobile telephones, a VoIP phone will not work if there is no power. Therelay switch of the present invention can be applied to a VoIP telephoneto detect occurrences of power outages. The relay switch of the presentinvention can the coupled to turn itself on to connect the VoIPtelephone to a regular phone line in the event of a power outage.

In the above description, the terms “source” and “drain” are used torefer to the current handling terminals of a MOS transistor. However,one of ordinary skill in the art would appreciate that the source anddrain terminals of a MOS transistor are interchangeable and thereference of a specific terminal as the source or the drain isillustrative only.

The above detailed descriptions are provided to illustrate specificembodiments of the present invention and are not intended to belimiting. Numerous modifications and variations within the scope of thepresent invention are possible. For example, the above-describedembodiments illustrate implementation of the relay switch circuit usingMOS transistors. One of ordinary skill in the art would appreciate thatthe circuits can be implemented using transistors of opposite polaritiesas long as the voltage signal polarities are modified accordingly.

Also, the above description describes a control signal generator forgenerating four control signals as two pairs of complementary signals.The two pairs of complementary signals are used to drive a mainconduction switch having two pairs of complementary conduction paths.One of ordinary skill in the art would appreciate that in otherembodiments, the control signal generator can generate one or morecontrol signals depending on the construction of the main conductionswitch of the relay switch. One of ordinary skill in the art wouldappreciate that the main conduction switch can be implemented using oneor more conduction paths.

Furthermore, the capacitors used in the above-described circuits operateas charge storage devices and other charge storage devices can also beused in place of the capacitors. Also, while the first and second latchcircuits 620, 640 provide useful functions in reinforcing the voltagelevels of control signals, the latch circuits are optional and may beomitted in other embodiments of the present invention. Additionally, inthe above-described embodiments, resistors, such as resistor R34, R8 andR9 are included in various circuits for setting the desired control forthe respective nodes. One of ordinary skill in the art would appreciatethat the resistors are optional elements and may be omitted in otherembodiments of the present invention.

The present invention is defined by the appended claims.

1. A method for operating a relay switch connected between a first dataport and a second data port and receiving a power supply voltage signal,the method comprising: measuring the voltage level of the power supplyvoltage signal; measuring an energy level of incoming signals on eitherone of the first and second data ports; opening the relay switch toisolate the first and second data ports when the voltage level of thepower supply voltage signal is greater than a predetermined supplyvoltage threshold level; opening the relay switch to isolate the firstand second data ports when the voltage level of the power supply voltagesignal is equal to or below the predetermined supply voltage thresholdlevel and the energy level of the incoming signals on either one of thefirst and second data ports is greater than a predetermined energythreshold level; and closing the relay switch to electrically connectthe first and second data ports when the voltage level of the powersupply voltage signal is equal to or below the predetermined supplyvoltage threshold level and the energy level of the incoming signals oneither one of the first and second data ports is less than or equal tothe predetermined energy threshold level.
 2. The method of claim 1,wherein the relay switch comprises a plurality of pairs of complementaryconduction paths connecting the wires of the first data port to thewires of the second data port, the method further comprises: shunting afirst conduction path in each pair of complementary conduction paths tothe power supply voltage signal when the relay switch is open; andshunting a second conduction path in each pair of complementaryconduction paths to a ground potential when the relay switch is open. 3.The method of claim 1, wherein the first data port and the second dataport each comprises a pair of wires carrying differential signals,measuring an energy level of incoming signals on either one of the firstand second data ports comprises: accumulating energy from firsttransitions of the incoming signals being carried on the pairs of wiresof the first data port and the second data port; and collecting chargeat a charge storage device at a first time constant, wherein the firsttime constant is selected to allow charge to accumulate at the chargestorage device when the incoming signals have a high pulse density. 4.The method of claim 3, wherein collecting charge at a charge storagedevice at a first time constant comprises: accumulating charge onto andremoving charge from the charge storage device at a rate determined bythe first time constant, a net charge accumulated on the charge storagedevice being a function of the pulse density of the incoming signals. 5.The method of claim 1, wherein the relay switch comprises a plurality ofpairs of complementary conduction paths connecting the wires of thefirst data port to the wires of the second data port, the method furthercomprises: generating one or more control signals in response to thevoltage level of the power supply voltage signal and the energy level ofthe incoming signals, each control signal of the one or more controlsignals driving a conduction path in the plurality of pairs ofcomplementary conduction paths; and opening the relay switch and closingthe relay switch in response to the one or more control signals.
 6. Themethod of claim 5, wherein generating one or more control signalscomprises: providing a plurality of pairs of complementary precursorcontrol signals, each precursor control signal corresponding to acontrol signal in the one or more control signals; generating at leastone precursor control signal in a pair of complementary precursorcontrol signals; and driving a control signal corresponding to the otherprecursor control signal in the pair of complementary precursor controlsignals to a complementary state.
 7. The method of claim 6, whereingenerating one or more control signals further comprises: driving thecontrol signals corresponding to the pair of complementary precursorcontrol signals through feedback to boost the voltage values of thecontrol signals.